1. Field of the Invention
The invention relates to methods for depositing high quality films of barium strontium titanate, referred to in the art as BST, on substrates, and apparatus for effecting such methods. Particularly, the invention relates to the fabrication of thin films of BST of suitable thinness and quality of use in integrated circuits.
2. Statement of the Problem
Barium strontium titanate is recognized as a material having potentially important uses in integrated circuits. See, "A Stacked Capacitor With (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256M DRAM" in IEDM(International Electron Devices Meeting) Technical Digest; December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. There are known methods for depositing thin films of BST. Sputtering, which is used in the above-cited references, may be the most common method. Other methods used include spin-coating. See G. M. Vest and S. Singaram, "Synthesis of Metallo-organic Compounds For MOD Powders and Films", Materials Research Society Symposium Proceedings, Vol. 60, 1986, pp. 35-42 and J. V. Mantese, A. L. Micheli, A. H. Hamdi, and R. W. Vest, "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", MRS BULLETIN, October 1989. With a sputtering process it is hard to control chemical proportions and stoichiometry of the materials deposited because the process is an inherently violent one and sputtering targets age with usage such that the stoichiometry of the resulting deposited films will vary with continuing target usage. Also, if a different film stoichiometry or a different compound is desired, a new target must be constructed and installed in the apparatus at considerable expense and inconvenience, noting that construction of new targets usually requires the expenditure of days or weeks of time. In the spin-coating papers referenced above, it was speculated that this process might have limited use for electronic devices, however, the grain size reported was 2000 .ANG. minimum, which is as large or larger than the film thickness typically used for capacitor dielectrics in integrated circuits, and the quality of the film was inferior to that produced by other processes, such as sputtering. Thus, up to the time of this invention, there was no known process for routinely making BST thin films of sufficient thinness and quality that they could be used in an integrated circuit.